Near-infrared spectroscopy (NIRS) is an optical spectrophotometric method of continually monitoring tissue oxygenation. The NIRS method is based on the principle that light in the near-infrared range (700 to 1,000 nm) can pass easily through skin, bone and other tissues but, within these wavelengths, hemoglobin has specific absorption spectra, dependent upon its oxidation state, i.e., oxygenated-hemoglobin (HbO2); and deoxygenated-hemoglobin (Hb). By using light sources that transmit near-infrared light at specific different wavelengths, and measuring changes in transmitted or reflected light attenuation, oxygenation concentration changes of HbO2 and Hb can be monitored.
Total hemoglobin is the summation of the two states of hemoglobin (Total Hb=HbO2+Hb), and is proportional to relative blood volume changes, provided that the hematocrit or hemoglobin concentration of the blood is unchanged. The most valuable aspect of NIRS is that it allows one to continually monitor cerebral oxygenation levels in an adult or neonate, especially in diseased conditions, in which oxygenation levels in the brain can be compromised, leading to brain damage or death.
It is known that near-infrared light passes through the skin and the skull of a neonate readily, and is absorbed by certain biological molecules in the brain. Near-infrared spectroscopy (NIRS) detects oxygenation changes in biological tissue (brain, muscle, or other organs) mainly at the micro circulation level (capillaries, arterioles, and venuoles) based on different absorption characteristics of the chromophores oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb) in the near-infrared spectrum (700-1,000 nm). Average tissue penetration is 2-3 cm with sub-second time resolution.
Another spectrophotometric method, called pulse oximetry, determines arterial oxygen saturation (SaO2) of peripheral tissue (i.e. finger, ear, nose) by monitoring pulsatile optical attenuation changes of detected light induced by pulsatile arterial blood volume changes in the arteriolar vascular system. The method of pulse oximetry requires pulsatile blood volume changes in order to make a measurement. Since venous blood is not pulsatile, pulse oximetry cannot provide any information about venous blood. Conversely, NIRS does not require pulsatile blood volume to calculate parameters of clinical value.
Relative changes of the concentrations of HbO2 and Hb can be quantified by using the modified Beer-Lambert Law, which takes into account the optical attenuation in a highly scattering medium like biological tissue. The modified Beer-Lambert Law can be expressed as:A=−log(I/I0)L=(åL×C×d×B)+G  (Equation 1);wherein A is the optical attenuation in tissue at wavelength L (units: optical density OD); I0 is the incident light intensity (units: Wlcm2); I is the detected light intensity; (L is the wavelength-dependent absorption coefficient of the chromophore (units: OD×cm−1×μM−1); C is the concentration of chromophore (units: μM); d is the light source-to-detector distance (units: cm); B is the light scattering differential path length factor (unitless); and G is a factor relating to tissue geometry and scattering of light (units: OD).
Absolute measurement of chromophore concentration is very difficult because G is unknown. However, over a reasonable measuring period of several hours to days, G remains constant, allowing for the measurement of relative changes of chromophore from a zero reference baseline. Thus, if time t2 is an arbitrary time after the start of the optical measurement at t1 (baseline), differential attenuation (A) can be calculated, canceling out the variables G and I0, providing that they remain constant. The objective is to determine changes in chromophore concentration [ΔC=C(t2)−C(t1)] from A derived from the equation:ΔA=−log(I2/I1)L=åL=ΔC×d×B  (Equation 2);
NIRS algorithms that are designed to calculate the relative changes of more than one chromophore use the multivariate form of Equation 2. To distinguish between, and to compute relative changes in, oxyhemoglobin (ΔHbO2) and in deoxyhemoglobin (ΔHb), a minimum of two different wavelengths, preferably from narrow spectral bandwidth light sources, like laser diodes, are preferred. The units of ΔHbO2 and ΔHb are in (moles per liter of tissue (μM) which is determined from a dimensional analysis of Equation 1.
It would be desirable to have a reusable NIRS transducer assembly having the ability to accurately control the energy level and size of a laser light field cast upon a subject's skin as well as improving the light detector signal-to-noise ratio by employing an improved EMI shielding scheme during use of the assembly. It would be desirable to combine light from multiple light sources into a single output fiber optic that is lightweight, and flexible, while providing sufficient light coupling efficiency. It would be desirable to have a transducer dislodgement-laser safety interlock system that would require no extra light source or detector components, while having the ability to disable laser operation due to transducer attachment failure and laser operation failure as well as a scheme that will verify secure transducer attachment before laser activation.